Accumulating load trolley and conveyor system

ABSTRACT

A conveyor system of the power-and-free type having an accumulation capability is provided. A trolley track extends in juxtaposition with an endless driving means and the relative spacing therebetween is varied to provide a drive region and an accumulation region. A load trolley for use in the system includes a first drive member having an unyieldable drive position relative to the trolley for driving engagement with the driving means and a second drive member having a yieldable drive position relative to the trolley. The relative positions of the first and second trolley drive members and the system driving means are such that the driving means drivingly contacts at least the unyieldable first drive member along the driving region of the trolley track and contacts only the second yieldable drive member along the accumulation region of the trolley track. The yieldable drive member is preferably a spring biased dog. Stop means selectively interposable in the trolley path at the accumulation region prevent advancement of the load trolley and the yieldable drive member yields to the increased force of the driving means to permit its passage relative thereto.

United States Patent 91 Winters 1451 Feb. 26, 1974 ACCUMULATING LOADTROLLEY AND CONVEYOR SYSTEM [75] Inventor:- Raymond Lewis Winters,Abilene,

Kans.

[73] Assignee: The Ehrsam Company, Abilene,

Kans.

[22] Filed: Aug. 17, 1972 [21] Appl. No.: 281,589

52 us. (:1 104/172 s, 104/96 51 1m. 01. F04b 3/00 58 Field of Search104/172 5, 96

[56] References Cited UNITED STATES PATENTS 3,664,267 5/1972 Di Rosa104/96 FOREIGN PATENTS OR APPLICATIONS 902,651 8/1962 Great Britain104/172 S Primary Examiner-Trygve M. Blix Attorney, Agent,0r-Firm-Stephen A. Schneeberger 5 7 ABSTRACT A conveyor system of thepower-and-free type having an accumulation capability is provided. Atrolley track extends in juxtaposition with an endless driving means andthe relative spacing therebetween is varied to provide a drive regionand an accumulation region. A load trolley for use in the systemincludes a first drive member having an unyieldable drive positionrelative to the trolley for driving engagement with the driving meansand a second drive member having a yieldable drive position relative tothe trolley. The relative positions of the first and second trolleydrive members and .the system driving means are such that the drivingmeans drivingly contacts at least the unyieldable first drive memberalong the driving region of the trolley track and contacts only thesecond yieldable drive member along the accumulation region of thetrolley track. The yieldable drive member is preferably a spring biaseddog. Stop means selectively interposable in the trolley path at theaccumulation region prevent advancement of the load trolley and theyieldable drive member yields to the increased force of the drivingmeans to permit its passage relative thereto.

20 Claims, 3 Drawing Figures PATENTEU F 25 m4 SHEET 2 OF 2 FIG. 2

FIG. 3

ACCUMULATING LOAD TROLLEY AND CONVEYOR SYSTEM BACKGROUND OF THEINVENTION The invention relates generally to conveyor systems and moreparticularly to conveyor systems providing an accumulating region forstorage. More particularly still, the invention relates to apower-and-free conveyor system and a load trolley used therein capableof providing accumulation and live storage of the load trolley.

In overhead power-and-free conveyor systems, an endless driving meansextends along a trolley track in juxtaposition therewith for drivingengagement with a load trolley supported on the trolley track.Engagement of the driving means with the load trolley is normallycontinuous; however, it may be desirable, from time to time, toaccumulate a plurality of loads at a work station and prevent theirfurther advancement for some period of time. While stopping of thedriving means would accomplish this function for a single load trolley,it would prevent the accumulation of a plurality of load trolleys andtherefore usually be unsatisfactory. In order to continue advancement ofsubsequent load trolleys to the accumulating region, it is generallydesirable that the driving means be in continuous operation, thusrequiring that the load trolley and the driving means be disengaged fromone another in some manner.

In most prior art conveyor systems having the abovementionedaccumulating capability, a dog member associated with the load trolleyand adapted for driving engagement with a dog member of the drivingmeans is constructed such that it may be withdrawn from the path of thedriving dog by means of a stop actuated lever associated with the loadtrolley. In such systems a probe or lever arm extends forward from thetrolley and is capable of withdrawing the trolley dog from engagementwith the driving means when it contacts and is actuated by a stop deviceinterposed in its path. Generally, the trolley dog is spring biased toits drive position and actuation of the lever arm by contact with thestop device serves to overcome the spring bias and positively withdrawthe trolley dog toward the trolley.

However, the dog-withdrawing lever arm described above must be preciselypositioned relative to the stop device to insure accurate contacttherewith for resulting actuation of the lever arm. Additionally, eachload trolley must be equipped with a member simulating theabove-described stop device extending rearward therefrom to actuate theforward extending lever arm of the next load trolley entering theaccumulation region. It will be evident that the opportunity formisalignment between a stop device and dog-withdrawing lever arm may besignificant. If actuation of the lever arm fails to occur, the trolleydog is not withdrawn and damage may occur to a stop device, the loadtrolleys and/or the driving means. Efforts to prevent such problems haveresulted in trolley dog-withdrawing mechanisms which are often large andrequire precise manufacturing, greatly adding to their cost.

SUMMARY OF THE INVENTION According to the present invention there isprovided a conveyor system comprising an endless driving means extendingalong a trolley track in juxtaposition therewith. The spacing betweenthe driving means and the trolley track is of a first distance along theregion of the trolley track at which the trolleys are to be normallydriven without accumulation and a second greater distance along theregion of the trolley track at which the trolleys are to be stopped andaccumulated.

Further, according to the invention, a load trolley is provided whichincludes a first drive member mounted thereon and having an unyieldabledrive position relative to the trolley and a second drive member mountedthereon and having a yieldable drive position relative to said trolley.The relative positions of the first and second trolley drive members andthe system driving means are such that the driving means drivinglycontacts at least the unyieldable first drive member along the drivingregion of the trolley track and contacts only the second yieldable drivemember along the accumulating region of the trolley track. The drivemember having the yieldable drive position is preferably a dog memberspring biased to a driving position, the biasing force of the springbeing overcome by a predetermined force applied to the drive member bythe driving means whereby the drive member yields relative to thetrolley and the driving means. Stop means are interposable in the pathof the load trolley at the accumulation region or station to preventadvancement of the load trolley. The resulting increase in force applied by the driving means to the yieldable drive member equals orexceeds that required to cause said member to yield, permitting passageof the drive means thereby.

BRIEF DESCRIPTION OF THE DRAWING F IG. 1 shows a small portion of aconveyor system including a load trolley of the invention shown at thedrive and accumulation regions of the system and showing elevationaldetail of the trolley in a cutaway sectional view at the accumulatingregion.

FIG. 2 is an elevational view of the load trolley shown in FIG. 1 of theinvention taken approximately along line 2-2 of FIG. 1.

FIG. 3 is similar to that of FIG. 1 and shows the accumulating loadtrolley in contact with the trolley stop device in the accumulatingregion.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a portion of an overhead power-and-free type of conveyorsystem. The system includes powered endless driving means, such asendless chain 10 having driving members such as pusher dog 14 extendingdownwardly therefrom at spaced locations and carried thereby. Endlesschain 10 may be supported by wheels 11 which are in turn supported bychain track 12. Chain track 12, without chain 10, is seen more clearlyin H6. 2. Pusher dog 14 may be rigidly secured to chain 10 as by pins 16or may be carried thereby in other manners, one of which is shown in U.S. Pat. No. 3,257,963 to D. M. King. The direction of travel of chain 10and associated pusher dog 14 is indicated by arrow 18. Trolleysupporting and guiding means such as trolley track 20 extend below chaintrack 12 substantially parallel thereto along most of its length and invertical alignment therewith. Trolley track 20 in the present embodimentcomprises a pair of opposed channel members sized and spaced toaccommodate and support a load trolley 22 therewithin. Trolley track 20,and more particularly the trolley supporting surface thereof, is spaceda first distance d, below pusher dog 14 along one region of the conveyorsystem and is spaced a second greater distance d below dog 14 alonganother region ofthe conveyor system. Spacing d of track 20 occursthroughout the conveyor system at the drive regions where positivedriving of load trolley 22 is intended with no provision for disengagingpusher dog 14 from the load trolley. Spacing d of track 2 occurs inthat, or those, regions of the conveyor system in which it is desired tostop load trolley 22 and permit relative motion of pusher dog 14therepast according to the invention. Such latter region may be referredto as an accumulating or storing region or station and will be ofsufficient track length to accommodate a plurality of load trolleys 22accumulated therealong. In one embodiment, d is 5/8 inch greater than11,. In the present embodiment, chain track 12 is substantially linearthrough those points at which trolley track 20 spacing changes between dand d and the spacing change is effected by ramps or inclines in thetrolley track. It will be appreciated that the spacing change might beeffected by reversing this arrangement of including moderate inclines inboth.

Accumulating load trolley 22 is adapted to be propelled by means ofpusher dog 14 on drive chain along trolley track in the direction 18.Trolley 22 includes a structural framework or body 24 having wheels 26mounted thereto for rolling support thereof by trolley track 20. Trolleybody 24 may be a pair of rigid plate members joined to one another as bywelding, and being spaced apart in the upper portion and including alower portion adapted for engagement of a load 28 therewith. Rollers 30may be affixed to frame 24 and positioned to maintain lateral alignmentof trolley 22 relative to track 20. Shafts 32 extend between the spacedapart upper plate members in the upper portion of trolley body 24 nearthe front and rear ends thereof. Shafts 32 include regions of polygonalcrosssection in mating engagement with correspondingly polygonallyshaped holes in the plate members of body 24, preventing rotation ofshafts 32 relative to said body. The ends of shafts 32 extend outwardlybeyond the plate members of body 24 and have wheels 26 mounted thereto,as by a retaining rivet engaging shaft 32, to allow rolling transport oftrolley 22. A bushing 34 surrounds each shaft 32 and extends between theinner surfaces of the plate members of trolley body 24 and serves tomaintain a constant spacing between said surfaces. Bushing 34 ispreferably of a resilient shock and sound absorbing material such aspolyurethane or nylon for reasons to become evident below.

In this embodiment, the dogs or abutments utilized for drivinglyengaging load trolley 22 with pusher dog 14, to drive the trolley, aremounted to body member 24 near its upper forward end and, according tothe invention, comprise two discrete dog members. Front retaining dog 40provides a nonyielding drive surface for contact with pusher dog 14along the conveyor system drive region. Accumulating dog 42 is capableof operation separately from retaining dog 40 and has a yieldable driveposition allowing it to drivingly engage pusher dog 14 in theaccumulating region of the system and to yield to the force of pusherdog 14 when trolley 22 is stopped, permitting dog 14 to pass.

As seen in FIG. 1, and more clearly in FIG. 2, front retaining dog 40comprises a pair of rigid members, such as metal stampings, laterallyspaced from one another in parallel relationship and joined by means ofconnecting member 44 extending therebetween and rigidly affixed thereto.Retaining dog 40 is pivotably mounted about a pin or shaft 46 whichextends transversely of travel arrow 18 generally horizontally betweenand supported by the plate members of body 24, in the upper forwardportion thereof. Retaining dog 40 is mounted to permit rearward orcounterclockwise rotation of the dog in the event of a rearwardlydirected force contacting the upper front surface thereof, but it isgravity biased in contact with front bushing 34 to limit forward orclockwise rotation. In this position of dog 40 against bushing 34, itsspaced plate members present a nonyielding rearward facing drive surfacewhich extends substantially vertically upward from trolley body 24.Front retaining dog 40 is of such height in its drive position thatpusher dog 14 easily contacts the drive surface thereof when the trolleytrack-topusher dog spacing is d,, but terminates below pusher dog 14 fortrolley track-to-pusher dog spacing d to prevent driving contact ofpusher dog 14 therewith.

Referring to FIGS. 1 and 2, an accumulating drive member such as dog 42is pivotally mounted on shaft 46 in the space occurring between thespaced-apart plate members of front retaining dog 40. The lateralspacing between dog member 42 and the plate members of dog 40 is fairlyclose, but sufficient to permit free pivotable motion of one relative tothe other. In this embodiment, dog 42 is preferably a rigid plate memberof the same material as that used in dogs 36 and 40. Dog 42 is so shapedrelative to its pivotable axis about shaft 46 that it may pivot orrotate forwardly, or clockwise, but is limited in rearward orcounterclockwise rotation by its contact with the bushing 34 at thefront of the trolley. Dog 42 is spring biased to the position in contactwith bushing 34 as will be described in greater detail below. In thisstop limited position, dog 42 presents a rearward facing drive surfacewhich extends substantially vertically upward from trolley body 24.Further, the drive surface is preferably vertically coplanar with thatof front dog 40. The drive surface portion of dog 42 extends upward fromtrolley 22 beyond the drive surface of dog 40 a sufficient distance, forinstance, 1/2 inch, to permit engagement of dog 42 with pusher dog 14when the increased trolley track-topusher dog spacing d exists. It ispreferable that the drive surface of pusher dog 42 additionally includea portion which is coplanar with that of dog 40 and is verticallycoextensive with at least a portion thereof. This will provide a drivesurface which is substantially continuous laterally from one platemember of dog 40 to the other to permit pusher dog 14 free lateralsliding motion across the drive surfaces provided by the front trolleydogs. In furtherence of such design, laterally extending tabs 48 of adurable material similar to that of dog member 42 are affixed to the dogat or near the upper end thereof and in coplanar relationship with thedriving surface thereof, thus also laterally extending the drive surfaceof dog 42 above dog 40.

Pusher dog 42 is biased to its drive position by spring 50. One end ofspring 50 is connected or anchored to dog 42, as by hooking the springend in hole 52 and the other end of the spring is anchored to trolleybody 24, as by anchoring it about pin 54 extending between thespaced-apart plates of the trolley body at a point preferably behind andbelow the location of anchor hole 52 when dog 42 is in its driveposition. Spring 50 is sized such that dog 42 is held in its driveposition against the driving forces of pusher dog 14 occurring whentrolley 22 is fully loaded and otherwise free to travel trolley track20. This would include sufficient retentive force by spring 50 tomaintain accumulating dog 42 in its drive position when trolley 22 isfully loaded and is transiting any upwardly inclined trolley tracksurfaces within the accumulation region of the conveyor system. Ingeneral, the accumulating trolley hereindescribed is intended for usewith loads of less than several hundred pounds.

Trolley stopping means, such as stop device 56, are supported in amanner not forming part of the invention and are disposed near thedownstream end of the accumulating region of trolley track relative tothe direction of travel 18. Stop device 56 may be any of a variety ofdevices well known in the art having an arm 58 or similar structurewhich is selectively interposable in the path of travel of trolley 22.Arm 58 is typically electrically or fluidly actuated from a trolleypassing position seen in FIG. 1 to a trolley stopping position seen inFIG. 3. Arm 58, in the trolley stopping position, will contact someportion of an oncoming trolley 22 and prevent the trolley from passingtherebeyond.

When arm 58 of stop device 56 is in the trolley stopping position, andtrolley 22 is in halted engagement therewith, as seen in FIG. 3, the netdriving force applied to accumulating dog 42 by pusher dog 14 increasessignificantly and spring 50 is sized to allow dog 42 to yield to suchforce and to pivot sufficiently forward and downward to permit pusherdog 14 to pass thereover in continued travel, as seen in FIG. 3.

in addition to dogs 40 and 42 positioned near the front of trolley 22and utilized for drivingly engaging pusher dog 14, a rear retaining dog60 may also be mounted near the upper rear end of trolley body 24 toprevent runaway or escape of trolley 22 from pusherdog 14 when trolleytrack 20 is inclined downward steeply relative to the forward speed ofthe pusher dog. Rear retaining dog 60 is generally similar to frontretaining dog 40 in side elevational view. However, unlike frontretaining dog 40 which includes an intermediate space occupied byaccumulating dog 42, rear retaining dog 60 is substantially continuouslaterally thereacross.

Typically, rear retaining dog 60 is comprised of several identicallyshaped metal stampings affixed closely adjacent one another as bywelding. Rear retaining dog 60 is pivotably mounted about shaft or pin62 which extends substantially horizontally between and is supported bythe spaced plate member of body 24 near the upper rear end thereof. Dog60 is mounted to allow forward or clockwise rotation about pin 62 whencontacted from the rear by pusher dog 14 at such time as trolley 22 isbeing initially engaged with the pusher dog. Dog 60 is gravity biased incontact with rear bushing 34 to limit its rearward or counterclockwisepivotal rotation. In this biased position of dog 60 against rear bushing 34, it presents a nonyielding, forward facing, laterally continuouscontact surface extending substantially vertically upward from trolleybody 24, similar to but in opposed relationship to front dog 40. Theheight of the contact surface of rear dog 60 may be essentially the sameas that of the drive surface of front retainer dog 40 as long as trolleytrack 20, in the accumulation region of the conveyor system, does notincline downwardly at an angle sufficiently steep to allow forwardescape of the trolley 22 from pusher dog 14. If it is necessary ordesirable to have a downwardly inclined trolley track section in thisregion exceeding the slope limit, rear retaining dog may extend upwardlyfrom trolley 22 a distance comparable to that of accumulating dog 42,thereby insuring that the trolley will be retained against escaping frompusher dog 14 in both the drive and the accumulation regions of theconveyor system, regardless of downward inclines in the trolley track.

In operation, the load trolley 22 of the accumulating conveyor system ofthe invention is propelled by engagement of pusher dog 14 with the drivesurfaces of both dogs 40 and 42 along the drive regions of the system atwhich the trolley track-to-pusher dog spacing is d,, seen in thelefthand side of FIG. 1. in the regions of the conveyor system in whichload trolleys 22 are to be accumulated such that their loads 28 might bestored or in some way manipulated, the trolley trackto-pusher dogspacing is increased to d This spaces nonyielding dog 40 below the zoneof possible contact with pusher dog 14 and continued propulsion of loadtrolley 22 is provided by contact of the pusher dog with the upper drivesurface of accumulating dog 42. In the event that a trolley 22 is totransit the accumulating region without stopping for accumulation, arm58 of stop device 56 remains in its trolley passing position andcontinued driving engagement between pusher dog 14 and accumulating dog42 propels the load trolley through the accumulating region to asubsequent drive region.

If it is desired to stop a load trolley 22 in the accumulation region,and possible accumulate others therebehind, arm 58 is interposed in thetravel path of the trolley, thereby stopping it. Dog 42 yields in aratchet-like manner to the increased force resulting from its immediatedriving pusher dog 14 and each subsequent pusher dog, thereby permittingcontinued motion of chain 10. Chain 10 continues to drivingly propelsubsequent trolleys 22 within the drive region and these trolleys will,in turn, be sequentially accumulated one behind another upon reachingthe conveyor system accumulating region. It may be necessary, due to thesize and geometry of the loads 28 to provide a contact arm, not shown,extending forwardly or rearwardly from trolley 22 to stop the nextsubsequent trolley before their loads 28 may contact one another. Itwill be appreciated that any such contact arm will not require theprecise positioning required by the dog-withdrawing lever arm of priorart accumulating trolleys.

Trolley track 20, throughout all or most of the accumulation region ofthe conveyor system, extends preferably horizontally or may be providedwith a shallow downward incline in the direction of chain travelindicated by arrow 18. Such downward incline may be desirable to preventrecoiling or rebounding of the last trolley 22 in an accumulated line oftrolleys each time its accumulating dog 42 recovers from yielding to anadvancing pusher dog 14. Further, pendulum-like oscillations of load 28may tend to cause rebounding of the last trolley accumulated as it isinitially stopped and the above-mentioned track incline will reduce oreliminate any significant rearward travel of the trolley in this region.Further to this end, forwardly ratcheting, antireverse stop means may beassociated with trolley track 20 at a position, or positions, rearwardlyof stop device 56 to limit any rearward travel of a trolley previouslystopped for accumulation.

While the load trolley of the preferred embodiment of the invention hasbeen described as having an accumulating dog which is a rigid memberpivotably mounted and spring biased to permit it to pivotably yield tocertain forces, the accumulating dog might comprise another form oflever device which is spring biased to its drive position. One examplewould be a leaf spring mounted at its lower end to the body of the loadtrolley and extending upwardly therefrom to provide the yieldable drivesurface required by the invention. Further, while the system has beendescribed as comprising an over and under chain and trolley trackarrangement, it will be appreciated that this spacial geometry may bevaried an might include a side-by-side arrangement in which spacings d,and d extended horizontally rather than vertically and the trolley drivedogs, both yieldable and nonyieldable, would extend horizontally outwardfrom the trolley.

It will be understood'that the embodiment shown and described herein ismerely illustrative and that changes may be made without departing fromthe scope of the invention as claimed.

What is claimed is:

1. A trolley comprising in combination, a frame, means for mounting saidframe for travel along a track, a first dog member extending from saidframe for drive engagement with a suitable driving means for moving saidtrolley in a particular direction, said first dog member beingunyieldable relative to said frame in said particular direction oftrolley movement and a second dog member extending from said frame toallow drive engagement therewith by said driving means independently ofsaid first dog member for moving said trolleyin said particulardirection, said second dog member being yieldable to permit passage ofsaid driving means relative thereto only on the application of apredetermined force of said driving means.

2. The trolley of claim 1 wherein said second dog member has at least aportion disposed outward of said first mentioned dog to form a drivesurface thereof.

3. A trolley comprising in combination, a frame, wheel members mountedon said frame, a first drive member having a fixed drive position fortransmitting driving force to said trolley in a particular direction anda second drive member also having a drive position for transmitting adriving force to said trolley in the same said particular direction andbeing yieldable with relation thereto upon the application of apredetermined force.

4. The trolley of claim 3 wherein said two drive members have coplanardrive surfaces.

5. The trolley of claim 4 wherein said yieldable drive member comprisesa lever device spring biased to its drive position.

6. The trolley of claim 5 wherein said yieldable drive member comprisesa pivotably mounted dog member, a dog stop member mounted on said frameand spring means acting between said frame and said dog member forbiasing said dog member to its drive position against said dog stopmember.

7. The trolley of claim 6 wherein the dog contacting surface of saidstop member is a resilient force absorb ing material.

8. The trolley of claim 4 wherein said first mentioned drive memberincludes a pair of parallel spaced plate members and said second drivemember is mounted intermediate said plate members.

9. The trolley of claim 8 wherein said yieldable drive member comprisesa lever device spring biased to its drive position.

10. The trolley of claim 9 wherein the drive surface of said seconddrive member extends outward of the drive surface of said pair ofparallel plate members.

11. A conveyor system comprising in combination:

a. trolley track means;

b. a load trolley supported by and adapted to move along said trolleytrack means in a predetermined path, said trolley including first dogmeans mounted thereon and having an unyieldable drive position andsecond dog means mounted thereon and having a drive position yieldablewith relation to said trolley upon application of a predetermineddriving force thereto;

0. a driving means adapted for movement along a path substantiallyparallel said trolley track along first and second regions thereof andincluding a pusher dog extending therefrom, said driving means beingspaced from said trolley track a first distance along said first regionthereof to permit driving contact between said pusher dog and said firsttrolley dog means and being spaced a second distance therefrom alongsaid second region thereof to permit contact between said pusher dog andonly said second trolley dog means; and

d. stop means interposable in the path of said trolley in said secondregion for applying a force to said trolley in a direction opposite thedirection of travel of said driving means and of a magnitude as great assaid predetermined force, whereby said second dog means drive positionyields, said pusher dog may move relative thereto and said trolley isprevented from advancing.

12. A conveyor system comprising in combination:

a. an endless driving means including dog members extending therefromand spaced therealong;

b. a trolley track means juxtaposed to said endless drive means andhaving at least one accumulating station spaced further from said drivemeans than the regions immediately before and after said station; and

c. a trolley mounted on said track means and including a first drivemember having an unyieldable drive position and a second drive memberhaving a yieldable drive position, said first member engaging saiddriving dog when said trolley is in a position on the track other thanthe accumulating station and said first member being free of saiddriving dog at said accumulating station, said second member engagingsaid driving dog at said accumulating station.

13. The conveyor system of claim 12 wherein said drive position of saidsecond drive member is yieldable relative to said trolley upon theapplication of a predetermined force applied thereto by said driving dogand further including trolley stop means interposable in the path ofsaid trolley at said accumulating station for opposing the advance ofsaid trolley such that said predetermined force is applied to saidsecond drive member by said driving dog, whereby said second drivemember drive position yields, said driving dog may advance relativethereto and said trolley is prevented from advancmg.

14. The conveyor system of claim 13 wherein said second drive membercomprises a lever device spring biased to its drive position.

15. The conveyor system of claim 14 wherein said trolley includes aframe and said second drive member comprises a pivotably mounted dogmember, a dog stop member mounted on said frame and spring means actingbetween said frame and said trolley dog member for biasing the latter toits drive position in contact with said dog stop member.

16. The conveyor system of claim 14 wherein said first and second drivemembers have drive surfaces coplanar with one another. I

17. The conveyor system of claim 16 wherein said driving means ispositioned above said trolley, said driving dog members dependingtherefrom and the drive surface of said second drive member extendsupwardly from said trolley beyond the drive surface of said first drivemember.

18. The conveyor system of claim 17 wherein a portion of the drivingsurface of said second drive member is vertically coextensive with aportion of the drive surface of said first drive member whereby saiddriving dog may simultaneously contact the drive surfaces of said firstand second drive members in positions along said track other than saidaccumulating station.

19. The conveyor system of claim 13 wherein said trolley track means isinclined downwardly in the direction of travel of said driving meansalong that length of said accumulating station rearward of said trolleystop means.

20. The conveyor system of claim 19 wherein said trolley furtherincludes a retaining member spaced rearwardly of said first and seconddrive members thereon and having a trolley retaining position fornonyieldably contacting a particular driving dog member to limit theforward movement of said trolley relative to said dog member.

1. A trolley comprising in combination, a frame, means for mounting saidframe for travel along a track, a first dog member extending from saidframe for drive engagement with a suitable driving means for moving saidtrolley in a particular direction, said first dog member beinguNyieldable relative to said frame in said particular direction oftrolley movement and a second dog member extending from said frame toallow drive engagement therewith by said driving means independently ofsaid first dog member for moving said trolley in said particulardirection, said second dog member being yieldable to permit passage ofsaid driving means relative thereto only on the application of apredetermined force of said driving means.
 2. The trolley of claim 1wherein said second dog member has at least a portion disposed outwardof said first mentioned dog to form a drive surface thereof.
 3. Atrolley comprising in combination, a frame, wheel members mounted onsaid frame, a first drive member having a fixed drive position fortransmitting driving force to said trolley in a particular direction anda second drive member also having a drive position for transmitting adriving force to said trolley in the same said particular direction andbeing yieldable with relation thereto upon the application of apredetermined force.
 4. The trolley of claim 3 wherein said two drivemembers have coplanar drive surfaces.
 5. The trolley of claim 4 whereinsaid yieldable drive member comprises a lever device spring biased toits drive position.
 6. The trolley of claim 5 wherein said yieldabledrive member comprises a pivotably mounted dog member, a dog stop membermounted on said frame and spring means acting between said frame andsaid dog member for biasing said dog member to its drive positionagainst said dog stop member.
 7. The trolley of claim 6 wherein the dogcontacting surface of said stop member is a resilient force absorbingmaterial.
 8. The trolley of claim 4 wherein said first mentioned drivemember includes a pair of parallel spaced plate members and said seconddrive member is mounted intermediate said plate members.
 9. The trolleyof claim 8 wherein said yieldable drive member comprises a lever devicespring biased to its drive position.
 10. The trolley of claim 9 whereinthe drive surface of said second drive member extends outward of thedrive surface of said pair of parallel plate members.
 11. A conveyorsystem comprising in combination: a. trolley track means; b. a loadtrolley supported by and adapted to move along said trolley track meansin a predetermined path, said trolley including first dog means mountedthereon and having an unyieldable drive position and second dog meansmounted thereon and having a drive position yieldable with relation tosaid trolley upon application of a predetermined driving force thereto;c. a driving means adapted for movement along a path substantiallyparallel said trolley track along first and second regions thereof andincluding a pusher dog extending therefrom, said driving means beingspaced from said trolley track a first distance along said first regionthereof to permit driving contact between said pusher dog and said firsttrolley dog means and being spaced a second distance therefrom alongsaid second region thereof to permit contact between said pusher dog andonly said second trolley dog means; and d. stop means interposable inthe path of said trolley in said second region for applying a force tosaid trolley in a direction opposite the direction of travel of saiddriving means and of a magnitude as great as said predetermined force,whereby said second dog means drive position yields, said pusher dog maymove relative thereto and said trolley is prevented from advancing. 12.A conveyor system comprising in combination: a. an endless driving meansincluding dog members extending therefrom and spaced therealong; b. atrolley track means juxtaposed to said endless drive means and having atleast one accumulating station spaced further from said drive means thanthe regions immediately before and after said station; and c. a trolleymounted on said track means and including a first drive member having anunyieldabLe drive position and a second drive member having a yieldabledrive position, said first member engaging said driving dog when saidtrolley is in a position on the track other than the accumulatingstation and said first member being free of said driving dog at saidaccumulating station, said second member engaging said driving dog atsaid accumulating station.
 13. The conveyor system of claim 12 whereinsaid drive position of said second drive member is yieldable relative tosaid trolley upon the application of a predetermined force appliedthereto by said driving dog and further including trolley stop meansinterposable in the path of said trolley at said accumulating stationfor opposing the advance of said trolley such that said predeterminedforce is applied to said second drive member by said driving dog,whereby said second drive member drive position yields, said driving dogmay advance relative thereto and said trolley is prevented fromadvancing.
 14. The conveyor system of claim 13 wherein said second drivemember comprises a lever device spring biased to its drive position. 15.The conveyor system of claim 14 wherein said trolley includes a frameand said second drive member comprises a pivotably mounted dog member, adog stop member mounted on said frame and spring means acting betweensaid frame and said trolley dog member for biasing the latter to itsdrive position in contact with said dog stop member.
 16. The conveyorsystem of claim 14 wherein said first and second drive members havedrive surfaces coplanar with one another.
 17. The conveyor system ofclaim 16 wherein said driving means is positioned above said trolley,said driving dog members depending therefrom and the drive surface ofsaid second drive member extends upwardly from said trolley beyond thedrive surface of said first drive member.
 18. The conveyor system ofclaim 17 wherein a portion of the driving surface of said second drivemember is vertically coextensive with a portion of the drive surface ofsaid first drive member whereby said driving dog may simultaneouslycontact the drive surfaces of said first and second drive members inpositions along said track other than said accumulating station.
 19. Theconveyor system of claim 13 wherein said trolley track means is inclineddownwardly in the direction of travel of said driving means along thatlength of said accumulating station rearward of said trolley stop means.20. The conveyor system of claim 19 wherein said trolley furtherincludes a retaining member spaced rearwardly of said first and seconddrive members thereon and having a trolley retaining position fornonyieldably contacting a particular driving dog member to limit theforward movement of said trolley relative to said dog member.